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Scientists Reset the Molecular Clock For Human Origins

Last week’s New Scientist cover story argued that a new consensus on dating the key stages in human evolution is in the offing.

According to correspondent Catharine Brahic:

This is one of the biggest and hardest questions in human evolution. We know that at some point we shared a common ancestor with chimps, but exactly when – and what that ancestor was like – have been maddeningly hard to pin down. Palaeontologists have searched for fossil remains, and geneticists have rummaged through the historical documents that are human and chimp DNA. Both made discoveries, but they did not see eye to eye.

No more. New estimates for when our lineage and chimps went their separate ways suggest that some of our established ideas are staggeringly wrong. If correct, they demand a rewrite of human prehistory, starting from the very beginning.

Well, ‘demand’ may be too strong a word in this case. A recent study of mutation rates in people living in Iceland showed that mutations accumulate in the genome at about half the rate that scientists first assumed: on average 36 mutations per child per generation. Augustine Kong of Decode Genetics in Reykjavik and his team published these findings in Nature this past August.

Assuming this rate is steady, the question arises how many generations can one go back in the timeline to estimate when the ‘break’ between the human and chimpanzee lineages took place.

According to his findings, on average, chimps reproduce when they are about 24.5 years of age. Using the new human mutation rate, Langergraber’s team estimated the human and chimpanzee lineages split at least 7 million years ago, but maybe even further back in the timeline, to as much as 13 million years.

That’s more than twice as long as initial estimates, which placed the human/chimp split between 4 and 6 million years ago.

Klaus Zuberbühler, a co-author on the paper from University of St Andrews in the UK, told New Scientist, “It’s clear that if this is right, most textbooks dealing with the history of our species will have to be rewritten. The significance can hardly be overestimated.”

Among key dates that will need to be revised: the human exodus from Africa, which was thought to be roughly 50,000 years ago based on initial genetic estimates, may now date back as far as 130,000 years ago.

For many paleontologists, this is welcome news, as the figure is consistent with the recent finding of human fossil remains in Israel that are believed to be 100,000 years old.

So, is it time to rewrite the textbooks?

According to paleontologist Chris Stringer of the London Museum of Natural History, it’s still too soon to say.

“This is an interesting time for ‘molecular clocks’,” he told me in an email, “but it is likely that different bits of the genome will be evolving at different rates, and we should be careful before generalizing to the whole genome.”

For example, Stringer cited a recent paper in Nature Genetics by MIT scientists highlighting contradictory data that suggests a faster mutation rate in some Icelandic populations.

Stringer also co-authored a 2009 paper examining date estimates for mitochondrial DNA that also suggest a faster, not slower, mutation rate.

Consider, too, a report this week of findings which pinpoint southern Africa as the home of the earliest technological and behavioral traditions of humans, 75,000 years ago. If some branch of humans left Africa 130,000 years ago, they may not have been as advanced as the cousins who stayed behind.

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Yes. One of the issues too that I find interesting is the discussion (or disagreements which is more often the case) that evolutionary biologists have over issues like the extent of natural selection’s role versus things like genetic drift.